1. Field of the Invention
The present invention relates to a four-wheel steering system in automotive vehicles, and more particularly to an electric control apparatus for the four-wheel steering system for controlling an actual steering angle of the rear road wheels in accordance with a steering mode selected by the driver and/or conditions of sensors detecting control factors for the rear road wheels.
2. Description of the Prior Art
In Japanese Patent Early Publication No. 59-81267, there has been proposed an electric control apparatus for a rear wheel steering mechanism in a four-wheel steering system, which is designed to calculate a steering angle of the rear road wheels on a basis of a control rule corresponding with a steering modes selected from a plurality of rear wheel steering modes thereby to steer the rear road wheels at the calculated steering angle. The electric control apparatus is further designed to delay operation speed of the rear wheel steering mechanism when the steering mode has been changed to another one of the steering modes thereby to moderate sudden change of the rear wheel steering angle for eliminating deterioration of stability of the vehicle in steering operation.
In the electric control apparatus, however, the actual steering angle of the rear road wheels may not correspond with the calculated steering angle when the latter steering angle has suddenly changed after change of the selected steering mode as shown by a chain line in FIG. 9. In this instance, the actual steering angle of the rear road wheels is varied as shown by a dotted line in FIG. 9. For this reason, the steering function of the rear road wheels at the newly selected mode may not be effected. As a result, stability of the vehicle in steering operation may not be obtained, and the driver feels incompatibility in his steering operation.